Process for fractionating water-containing crude amine mixtures from amine synthesis

ABSTRACT

A mixture containing mixture one or more amines, water, low-boilers and high-boilers, is fractioned in a process wherein 
     (i) low-boilers are separated from the mixture by distillation, 
     (ii) high-boilers are separated from the mixture by distillation, 
     (iii) the mixture is extracted with a sodium hydroxide solution to form an aqueous, sodium-hydroxide-containing first phase and an aqueous-organic, amine-containing second phase, and 
     (iv) the aqueous-organic second phase is distilled to form an amine/water azeotrope and an essentially anhydrous amine, and the amine/water azeotrope is recycled to the extraction step (iii).

The invention relates to a process for fractionating a mixture whichcomprises one or more amines, water, low-boilers with or withouthigh-boilers.

In the reaction of ammonia, primary or secondary amines with alcohols orwith aldehydes in the presence of hydrogen, a reaction product produced,inter alia, is water, which frequently forms an azeotropic amine/watermixture with the product amine formed. In addition, the product mixturecomprises low-boilers having a lower boiling point than that of theamine/water azeotrope, for example unreacted ammonia, or starting amine,and high-boilers having a higher boiling point than that of the productamine, for example higher-molecular-weight byproducts.

GB 1,102,370 describes a process for extractive distillation of anethylenediamine/water mixture in which, in a first distillation column,the ethylene/water crude mixture is evaporated and the ascending vaporis brought into contact with aqueous sodium hydroxide solution flowingin countercurrent. At the top of the first column a low-waterethylenediamine/water mixture is obtained which has an amineconcentration above the azeotropic point, which mixture is furtherdistilled by simple rectification in a second column. At the top of thesecond column, pure ethylenediamine is obtained and at the bottom of thesecond column an ethylenediamine/water mixture is obtained which iscombined with the crude mixture and recycled to the extractivedistillation.

DE-A 29 02 302 describes a process for separating ethylarnine mixturesin which a diethylamine-, triethylamine-, ethanol-, water- and possiblymonoethylamine-containing mixture is extracted with water and awater-immiscible solvent, an aqueous phase and a water-immiscible phasebeing obtained. The two phases are separated and further worked up bydistillation. Water-immiscible solvents used are n-butane, n-hexane andcumene.

DE-A 27 23 474 describes a process for fractionating a water-,monoethylamine-, diethylamine- and triethylamine-containing mixture, inwhich an essentially anhydrous mixture of monoethylamine anddiethylamine and triethylamine is separated off by distillation and, bydistillation, monoethylamine is separated off from the anhydrous mixtureof monoethylamine and diethylamine.

EP-A 0 881 211 describes a process for preparing anhydrous2-amino-1-methoxypropane in which, in an extraction step, a2-amino-1-methoxypropane-containing aqueous reaction mixture is admixedwith sodium hydroxide solution, forming a sodium-hydroxide-containingaqueous phase and a 1-amino-1-methoxypropane-containing phase, theaqueous phase is separated off and, in a distillation step, the2-amino-1-methoxypropane-containing phase is distilled, an azeotrope ofwater and 2-amino-1-methoxypropane first being produced, which isrecycled to the extraction step, and then anhydrous2-amino-1-methoxypropane then being produced.

It is an object of the present invention to provide an improved processfor fractionating water-containing crude amine mixtures from aminesynthesis, which process is suitable for a multiplicity of differentcrude amine mixtures.

We have found that this object is achieved by a process forfractionating an amine-containing mixture which comprises one or moreamines, water, low-boilers and high-boilers, having the steps (i) to(iv):

(i) separating off by distillation low-boilers from the amine-containingmixture,

(ii) separating off by distillation high-boilers from theamine-containing mixture,

(iii) extracting the amine-containing mixture with sodium hydroxidesolution, producing an aqueous, sodium-hydroxide-containing first phaseand an aqueous-organic, amine-containing second phase,

(iv) distilling the aqueous-organic second phase, producing and anessentially anhydrous amine, an amine/water azeotrope and recycling theamine/water azeotrope to the extraction step (iii).

Surprisingly, separating off high-boilers before carrying out extractionstep (iv) avoids the unwanted formation of solids in the extractor. Theinventive process is preferably carried out continuously.

The inventive process can be carried out in such a manner that, in thedistillation step (iv), the amine/water azeotrope is produced first andthen the essentially anhydrous amine is produced. “Amine” is also takento mean a mixture of a plurality of amines.

In a preferred embodiment, in distillation step (iv), the amine/waterazeotrope is produced as sidestream takeoff in the enrichment part ofthe distillation column and recycled to the extraction step (iii), andthe essentially anhydrous amine is produced as sidestream takeoff in thestripping part of the distillation column, further low-boilers areproduced as overhead takeoff and further high-boiler-containing amine isproduced as bottom-phase takeoff.

The further high-boiler-containing amine produced as bottom-phasetakeoff is preferably recycled to step (ii).

In a subsequent distillation step (v) the essentially anhydrous amineproduced in step (iv) can be further fractionated.

In a further preferred embodiment, in distillation step (iv), theamine/water azeotrope is produced as sidestrearn takeoff in theenrichment part of the column and recycled to the extraction step (iii),and further low-boilers are produced as overhead takeoff and theessentially anhydrous amine is produced as bottom-phase takeoff.

In a subsequent distillation step (v), the essentially anhydrous amineproduced in step (iv) can be further fractionated.

Removing a low-boiler fraction as overhead takeoff from the distillationcolumn in step (iv) avoids accumulation of low-boilers due to recyclingamine/water azeotrope to extraction step (iii) in the continuousprocedure.

A downstream distillation step (v) is required to produce the pureamines, if the starting mixture comprises two or more amines which, withwater, form azeotropes having very similar boiling points. Examples ofthis are azeotropes having boiling points which differ by no more than10° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 schematically illustrate two embodiments of the inventionwhich are described in more detail below.

With reference to FIG. 1, the reaction output of amine preparation whichis to be fractionated is fed as inflow stream 1 to a low-boiler removalcolumn a. Low-boilers are, for example, unreacted starting materialamine. The low-boiler removal column is generally operated at a pressureof from 1 to 40 bar absolute, preferably from 10 to 30 bar absolute, andat a temperature of generally from −20 to 300° C., preferably from 30 to250° C. The number of theoretical plates is generally from 3 to 80,preferably from 10 to 30.

At the top of the low-boiler columna, a majority of the low-boilers areproduced as overhead takeoff stream 2, which can be recycled to theamine synthesis. The bottom-phase takeoff stream 3 is fed to thehigh-boiler removal column b having generally from 3 to 80, preferablyfrom 10 to 30, theoretical plates and distilled at a pressure ofgenerally from 0.15 to 40 bar absolute, preferably from 1 to 5 barabsolute, and at a temperature of generally from −20 to 300° C.,preferably from 30 to 250° C. As bottom-phase take off stream 4,high-boilers are produced which are discharged from the process.High-boilers are, for example, byproducts having a higher molecularweight than the desired product amines. As overhead takeoff stream 5 anamine/water azeotrope is produced which still comprises traces oflow-boilers and high-boilers. As a result of the high-boiler removal,the solids formation does not occur in the downstream extractor c.

The overhead takeoff stream 5 is combined with the sidestream takeoffstream 9 of the azeotrope removal column d and fed to the extractor c.The extractor c can be of single or multistage design. A single-stageextractor c is, for example, a single mixer-settler extractor.Multistage extractors c are, for example, an extraction column orextractor cascade. Suitable extraction columns are, for example, packedcolumns, sieve-plate columns, cascade columns, pulsed columns, rotarycolumns and centrifugal columns. An extractor cascade is, for example, aplurality of series-connected mixer-settler extractors which can also beconstructed in a space-saving manner as tower extractors or boxextractors. Preferably, the extractor c is multistage, particularlypreferably a countercurrent flow extraction column having generally from1 to 25, preferably from 4 to 10, theoretical plates. These aregenerally operated at a pressure at which all components of theextraction mixture are above their boiling point. The temperature isselected in such a manner that none of the components of the extractionmixer is above its boiling point, and, in addition, a viscosity of bothphases is established at which dispersion of the two phases is possiblewithout problem. The temperature is generally from 5 to 200° C.,preferably from 20 to 70° C., for example from 40 to 50° C. Sodiumhydroxide solution is added as inflow stream 6. Generally, theconcentration of the sodium hydroxide solution is from 1 to 75% byweight, preferably from 25 to 50% by weight. After phase separation theaqueous, sodium-hydroxide-containing phase is discharged from theprocess as effluent stream 7.

The aqueous-organic, amine-containing phase is fed as stream 8 to theazeotrope separation column d. The azeotrope separation column generallyhas from 3 to 80, preferably from 10 to 30, theoretical plates and isoperated at a pressure of generally from 1 to 40 bar, preferably from 2to 8 bar, and at a temperature of from −20 to 300° C., preferably from50 to 120° C. In the enrichment part of this column an amine/waterazeotrope is obtained as sidestream takeoff stream 9 and is combinedwith the overhead takeoff stream 5 of the high-boiler removal. Furtherlow-boilers are obtained as overhead takeoff stream 10. As bottom-phasetakeoff stream 11, anhydrous amine is obtained which can still comprisetraces of high-boilers. The bottom-phase takeoff stream 11 can befurther distilled in the distillation column e, with pure amine beingobtained as overhead takeoff stream 12 and other high-boilers asbottom-phase takeoff stream 13.

With reference to FIG. 2, a variant of the abovedescribed process isdescribed. In this case, anhydrous amine is not taken off asbottom-phase takeoff stream from the azeotrope separation column, but assidestream takeoff stream 12. As bottom-phase takeoff stream 11, anamine having an elevated content of high-boilers is taken off, combinedwith the bottom-phase takeoff stream 3 of the low-boiler removal andrecycled to the high-boiler removal. Preferably, the bottom-phasetakeoff stream 11 is from 0.1 to 20% by weight of the total of streams11 and 12.

The sidestream takeoff stream 12 can be further fractionated in adownstream purifying distillation column. This is necessary to producepure amines if two or more amines are present in the starting mixture,which form with water an azeotrope having very similar boiling points.Examples of such mixtures areN-methylmorpholine/N-ethylmorpholine/water,pyrrolidine/N-methylpyrrolidine/-water andpiperidine/N-methylpiperidine/water.

The starting mixtures to be fractionated by the process according to theinvention can vary greatly in their composition and generally comprise,per mol of product amine, from 0 to 9 mol, preferably from 0 to 3 mol,of low-boilers, from 1 to 10 mol, preferably from 1 to 4 mol, of waterand, based on the total of all components of the starting mixture, from2 to 20% by weight of high-boilers. Examples of starting mixtures to befractionated by the process according to the invention are:

the product mixtures produced in the preparation of dimethylpropylaminefrom dimethylamine and propionaldehyde in the presence of hydrogen, orfrom dimethylamine and propanol. These can comprise, as low-boilers,unreacted dimethylamine and, formed from this by disproportionation,monomethylamine and trimethylamine. If the preparation starts frompropionaldehyde, the starting mixture can comprise, as high-boilers, thecompounds 2-methylpent-2-enaldehyde, 2-methylvaleraldehyde,dimethyl-(2-methylpent-2-enyl)amine and dimethyl(2-methylpentyl)aminewhich are formed by the reaction of two molecules of propionaldehyde andsubsequent amination and/or hydrogenation.

the product mixture produced in the preparation of piperidine from1,5-pentanediol and ammonia. This mixture can comprise unreacted ammoniaas low-boiler. High-boilers which can be present are2-methylpentanediol, dipiperidinylpentane and 2-methylpiperidine.

the product mixture produced in the preparation of N-methylpiperidinefrom 1,5-pentanediol and methylamine. As low-boilers, all of theabovementioned methylamines may be present. High-boilers are, forexample, the reaction products of 1,5-pentanediol with one or twomolecules of dimethylamine.

the product mixture obtained in the preparation of morpholine fromdiethylene glycol and ammonia. The low-boiler is ammonia, and thehigh-boiler is, for example, dimorpholino diglycol.

the product mixture obtained in the preparation of N-methylmorpholinefrom diethylene glycol and methylamine. Low-boilers which may be presentare all of the abovementioned methylamines. High-boilers are, forexample, the reaction products of diethylene glycol with one or twomolecules of dimethylamine.

the product mixture produced in the preparation of pyrrolidine from1,4-butanediol and ammonia. The low-boiler is ammonia, high-boilers are,for example, the products formed by further reaction of the resultantpyrrolidine with unreacted 1,4-butanediol.

the product mixture produced in the preparation of N-methylpyrrolidinefrom 1,4-butanediol and methylamine. Low-boilers which may be presentare all of the abovementioned methylamines. High-boilers are, forexample, the reaction products of 1,4-butanediol with one or twomolecules of dimethylamine.

The invention is described in more detail by the example below.

EXAMPLE

The product mixture obtained in the synthesis of piperidine from ammoniaand pentanediol is treated by the process according to FIG. 1. The massflow rate and composition of the starting mixture (stream 1) and of theother streams occurring in the workup are reported in the table below.The mixture is distilled at 21 bar in a low-boiler removal column having22 theoretical plates. At the top of the column ammonia is taken off(stream 2). The bottom temperature is 204° C. The top temperature is 46°C. The bottom-phase discharge of the low-boiler column (stream 3) isthen distilled at 1 bar in a high-boiler removal column having 25theoretical plates. At the bottom a stream is taken off (stream 4) whichprincipally comprises dipiperidinylpentane, 2-methylpiperidine and2-methylpentanediol. The overhead takeoff (stream 5) comprises water,piperidine and 2-methylpiperidine. The bottom temperature is 176° C. Thetop temperature is 95° C. The overhead takeoff stream of the high-boilerremoval column is combined with the sidestream takeoff stream (stream 9)of the azeotrope distillation column and fed to the extraction columnoperating at atmospheric pressure and having 10 theoretical plates. Theextraction is carried out at atmospheric pressure with 50% strength byweight sodium hydroxide solution which is added at the top of theextraction column. The bottom temperature is 55° C. The top temperatureis 39° C. As organic phase, a low-water mixture of piperidine and2-methylpiperidine, which has a residual water content of 2% by mass, istaken off at the top of the extraction column (stream 8), which is fedto the azeotrope distillation column having 18 theoretical plates. Thisoperates at atmospheric pressure. The bottom temperature is 113° C. Thetop temperature is 95° C. At the third plate from the top, the azeotropewater/piperidine or water/2-methylpiperidine (stream 9) is taken off inthe sidestream takeoff and combined with the overhead takeoff stream ofthe high-boiler removal. The bottom-phase takeoff stream (stream 11) ofthe azeotrope distillation column is fed to the purifying distillationcolumn operating at atmospheric pressure. At the top of the purifyingdistillation column there is produced the product stream whose maincomponent is piperidine (stream 12). At the bottom a stream (stream 13)is separated off which principally comprises 2-methylpiperidine. Thebottom temperature is 123° C. The top temperature is 109° C. The processwas run over a period of 40 days without solids formation in theextraction column and without accumulation of low-boilers.

TABLE Stream* 1 2 3 4 5 8 9 10 11 12 13 Water 1.136 0.000 1.136 0.0001.136 0.054 0.050 0.004 0.001 0.001 0.000 NH₃ 2.755 2.755 0.000 0.0000.000 0.000 0.000 0.000 0.000 0.000 0.000 Piperidine 2.507 0.000 2.5070.003 2.505 2.606 0.105 0.009 2.492 2.480 0.012 2-methyl- 0.104 0.0000.104 0.104 0.000 0.000 0.000 0.000 0.000 0.000 0.000 pentanediol2-methyl- 0.084 0.000 0.084 0.044 0.040 0.040 0.000 0.000 0.040 0.0030.037 piperidine Dipiperidinyl- 0.098 0.000 0.098 0.098 0.000 0.0000.000 0.000 0.000 0.000 0.000 pentane Total 6.683 2.755 3.028 0.2483.680 2.700 0.155 0.013 2.532 2.483 0.049 *Figures in kg/h

What is claimed is:
 1. A process for fractionating an amine-containingmixture comprising at least one amine, water, low-boilers andhigh-boilers, wherein said process comprises (i) separating off bydistillation low-boilers from the amine-containing mixture, and (ii)separating of f by distillation high-boilers from the amine-containingmixture, to produce a pretreated amine-containing mixture, and (iii)extracting the pretreated amine-containing mixture with sodium hydroxidesolution to form an aqueous, sodium-hydroxide-containing first phase andan aqueous-organic, amine-containing second phase, (iv) distilling theaqueous-organic second phase in a distillation column having anenrichment part and a stripping part to form an amine-water azeotrop andan essentially anhydrous amine, and recycling the amine-water azeotropto the extraction step (iii), and wherein, in step (iv), the amine-waterazeotrop is obtained as a sidestream takeoff in the enrichment part ofthe distillation column and is recycled to the extraction step (iii),and the essentially anhydrous amine is obtained as a sidestream takeoffin the stripping part of the distillation column, further low-boilersare obtained as an overhead takeoff and further high-boiler-containingamine is obtained as a bottom phase takeoff.
 2. A process as claimed inclaim 1, wherein the further high-boiler-containing amine produced instep (iv) is recycled to step (ii).
 3. A process as claimed in claim 1,wherein the essentially anhydrous amine produced in step (iv) is furtherfractionated by distillation in a subsequent step (v).
 4. A process asclaimed in claim 1, wherein the extraction step (iii) is carried out inmultiple stages.
 5. A process as claimed in claim 1, wherein thedistillation column employed in step (iv) has from 3 to 80 plates andthe distillation is conducted at a pressure of from 1 to 40 bar and at atemperature of from −20 to 300° C.
 6. A process as claimed in claim 1,wherein the mixture to be fractionated is selected from the groupconsisting of product mixtures obtained in the manufacture ofdimethylpropylamine from dimethylamine and propionaldehyde and hydrogen,of dimethylpropylamine from dimethylamine and propanol, of piperidinefrom 1,5-pentanediol and ammonia, of N-methylpiperidine from1,5-pentanediol and methylamine, of morpholine from diethylene glycoland ammonia, of N-methylmorpholine from diethylene glycol andmethylamine, of pyrrolidine from 1,4-butanediol and ammonia, and ofN-methylpyrrolidine from 1,4-butanediol and methylamine.
 7. A process asclaimed in claim 1, which is carried out continuously.